This invention relates to a cast metal-matrix composite material, and, more particularly, to a chemical modification to the matrix of such a material that improves its microstructure.
Reinforced metal matrix composite materials have gained increasing acceptance as structural materials. Metal matrix composites typically are composed of reinforcing particles such as fibers, grit, powder or the like that are embedded within a metallic matrix. The reinforcement imparts strength, stiffness and other desirable properties to the composite, while the matrix protects the reinforcement and transfers load within the composite piece. The two components, matrix and reinforcement, thus cooperate to achieve results improved over what either could provide on its own.
Twenty years ago, reinforced composite materials were little more than laboratory curiosities because of very high production costs and their lack of acceptance by product designers. More recently, great advances in the production of nonmetallic composite materials, such as graphite-epoxy composite materials, have been made, with a significant reduction of their cost. During that period, the cost of metal-matrix composite materials remained relatively high.
In the last several years, the discovery of a processing technology that permits the reproducible production of large quantities of cast reinforced composite materials with metal matrices has significantly reduced the cost of these materials. See, for example, U.S. Pat. No. 4,759,995 and U. S. Pat. No. 4,786,467, whose disclosures are incorporated by reference.
Since the discovery of the methods of the '995 and '467 patents, many applications for such materials have been developed, and their volume of use has increased significantly so that they have become a major new class of structural material. These cast metal matrix composite materials offer the property improvements of composite materials at little more than the cost of conventional monolithic materials. Even with recent cost reductions, nonmetallic matrix composite materials remain significantly more costly to produce than monolithic materials and the cast composite materials. The cast composite materials may be used at elevated temperatures or under other conditions that preclude the use of nonmetallic matrix composite materials.
However, it has been found that in some instances the microstructures of the metal matrix composite materials produced by casting include various types of irregularities that interfere with their post-casting fabrication and use. For example, agglomerations of reinforcement particles with other solids have sometimes been observed in the matrix of the cast, solidified material. The agglomerations cause reductions in the general property levels of the composite material due to the reduction in the reinforcement level in other regions and increased inhomogeneity of the structure, and also can be the sites for the initiation of premature failure of the composite material in loading.
There exists a need for a modification to the preparation of cast composite materials that reduces microstructural irregularities, and results in a more uniform structure. The present invention fulfills this need, and further provides related advantages.